1. Field of Invention
This invention relates to induced current linear and rotary position transducers.
2. Description of Related Art
Various induced current position transducers are known. U.S. Pat. No. 5,973,494, incorporated herein by reference in its entirety, discloses an electronic caliper using an induced current position transducer. U.S. Pat. No. 6,005,387, incorporated herein by reference in its entirety, discloses various reduced-offset high accuracy induced current position transducers and associated signal processing techniques. U.S. application Ser. No. 09/421,497, now U.S. Pat. No. 6,400,138, incorporated herein by reference in its entirety, discloses various reduced-offset high accuracy induced current absolute position transducers and associated signal processing techniques.
A reduced-offset-type induced current position transducer generally includes a transmitter winding, a corresponding receiver winding and a signal generating and processing circuit on a readhead. Reduced-offset-type induced current position transducers are often configured so that the transmitter winding and the corresponding receiver winding occupy separate regions on the readhead. The transmitter winding and the corresponding receiver winding are spaced apart in a direction transverse to the measuring axis of the position transducer. The transducer also includes a scale having at least one scale loop. The transmitter winding is inductively coupled to a first portion of the scale loop and a second portion of the scale loop is, in turn, inductively coupled to the corresponding receiver winding.
When a time-varying signal originating from the signal generating and processing circuit passes through the transmitter winding, a primary magnetic field is generated. The transmitter winding is inductively coupled to the first portions of the scale loops by the primary magnetic field. The second portions of the scale loops generate secondary magnetic fields. The receiver winding is inductively coupled to the second portions of the scale loops by the secondary magnetic fields.
At least one of the transmitter winding or the receiver winding is formed in a periodic pattern, such as a sinusoidal pattern, having dimensions corresponding to the coupling loops. Various winding configurations are known in the art to reduce the extraneous inductive coupling in the device. The receiver windings inductively couple with the second loop portions of the scale loops in differing degrees, depending on the position of the scale relative to the readhead.
U.S. Pat. No. 6,011,389, incorporated herein by reference in its entirety, discloses an incremental induced current position transducer. U.S. Pat. Nos. 5,804,963, 4,853,684 and 6,259,249 disclose various other types of induced current position transducers. The ""389, ""963, ""684 and ""249 patents include scale configurations wherein periodically arranged scale elements, in some embodiments, comprise conductive loops. However, these patents include no teaching directed toward the size of the conductors comprising the conductive scale loops.
Furthermore, the conductor sizes represented in the drawings are inconsistent and/or artifacts of the illustration process. Furthermore, the transmitter and receiver windings of the aforementioned patents are either not spaced apart in a direction transverse to the measuring axis of their respective induced current position transducers, or their underlying operating principle is otherwise significantly different from that associated with the present invention. For at least these reasons, the ""389, ""963, ""684 and ""249 patents are not instructive regarding scale loop designs according to this invention.
It is desirable for the position measurements obtained with reduced-offset-type induced current position transducers to have improved accuracy and resolution relative to the accuracy and resolution that are possible with known devices.
The accuracy and resolution of measurements taken with reduced offset-type induced position transducers are generally related to the signal strength provided by the transducer and the related signal-to-noise ratio of the overall position transducer signal processing. The signal strength is, in turn, related to the efficiency of the scale loops in inductively coupling to the transmitter and receiver windings of the reduced-offset-type induced current position transducer. Therefore, the coupling efficiency of the scale loops is an important factor in such transducer designs.
The resolution of measurements taken with reduced-offset-type induced current position transducers is improved when the spatial period or wavelength of the spatially varying magnetic field created by the scale loops is short, and when the signal measured by the receiver has a high signal-to-noise ratio. However, as the spatial period or wavelength of the magnetic field created by the scale loops is shortened by reducing the dimensions of the scale loops along the measuring axis, the signal provided by the scale loops is generally weaker at a given operating distance away from the scale loops. Further, fabricating the scale generally becomes more expensive as the dimensions of the scale loops are reduced.
Accordingly, there is a need for improved scale loop designs that help to improve performance and reduce fabrication costs of induced current position transducers.
The signal-to-noise ratio of the signal produced by the scale loops in a reduced-offset-type induced current position transducer depends significantly on the efficiency of the transfer of energy through the scale loops. The efficiency of the transfer of energy through the scale loops generally improves by decreasing the resistance and increasing the inherent inductance of the scale loops. These factors can be affected by manipulating the layout of the scale loops and the cross-sectional dimensions of the scale traces that individually form each of the scale loops.
However, scale loop design constraints are imposed by factors such as the desired scale wavelength and the desired overall dimensions of the scale and the readhead of the reduced offset-type induced position transducer. In particular, when a desired design constrains the distance spanned by a scale loop along the measuring axis, a balance must be achieved between the width of the scale trace, which is representative of the resistance,and the interior area of the scale loop, which is representative of the inherent inductance. Thus, an optimum scale loop design must consider the relationship between these parameters, as well as other practical fabrication considerations.
The resistance of a scale loop, is dependent on the geometry of the scale trace from which the scale loop is formed. In particular, the resistance of the scale loop is dependent on the thickness and width of the scale trace, as well as its length. Additionally, the effective resistance of an operating scale loop also depends on the skin depth of the scale trace. The skin depth, in turn, depends on the material from which the scale trace is formed, and the operating frequency of the reduced offset-type inductive position transducer.
The inherent inductance of a scale loop depends primarily on the interior area of the scale loop. In particular, as the interior area of a scale loop is reduced, the inherent inductance of the scale loop is also reduced. As previously discussed, when the distance spanned by a scale loop along the measuring axis is constrained by other design imperatives, the interior area, and thus the inherent inductance, of a scale loop necessarily decreases as the width of the scale trace. Thus, to improve the related performance of the scale loops in a reduced offset-type induced current position transducer, the width of the scale trace forming the scale loop should be chosen to balance the need to minimize the resistance against the need to maintain a high inherent inductance.
Accordingly, this invention provides efficient scale loops that provide increased signal strength and improved resolution and accuracy of measurements, when employed in reduced offset-type induced current position transducers.
This invention separately provides scale loops of reduced-offset-type induced current position transducers, having reduced electrical resistance.
This invention separately provides scale loops of reduced-offset-type induced current position transducers, having improved inherent inductance in relation to their resistance.
Scale loops for reduced-offset induced current position transducers are provided which allow high resolution position measurements to be made. The scale loops designed according to this invention include a scale trace formed of an electrically conductive material arranged in a loop that is continuous and electrically isolated from adjacent scale loops. The shape of the scale loop is not limited, but rather is determined by the particular configuration of the reduced offset-type induced current position transducer in which the scale loop will be employed.
These and other features and advantages of this invention are described in, or are apparent from, the following detailed description of various exemplary embodiments of the systems and methods according to this invention.